With recent trends toward digitization of household electrical appliances, an increase in the speed of CPUs, and the like, issues relating to an electromagnetic disturbance that adversely affects the peripheral devices and human bodies have been attracting attention. To address the issues, “The Voluntary Control Council for Interference by Information Technology Equipment (VCCI)” was established in Japan. To comply with the rules of VCCI, there has been an increasingly strong trend in the industry toward voluntary control in terms of the issues relating to an electromagnetic disturbance. To address electromagnetic noise generated from electronic boards inside electric and electronic devices, for example, there is a technique of enclosing the electronic boards with a shield box composed of a metal (electrically conductive) material to achieve electromagnetic shielding.
In a shield box, the electrically conductive material constituting the shield box reflects electromagnetic waves to achieve electromagnetic shielding. As the electrical conductivity of a material constituting a shield box increases, the reflectivity of electromagnetic waves increases and the electromagnetic shielding property improves. Accordingly, to ensure the electromagnetic shielding property of a shield box, it is important that metal plates constituting the shield box have high electrical conductivity.
Such a shield box is produced by shaping metal plates and hence has discontinuous portions (joints and joining portions). Leakage or entry of electromagnetic waves tends to occur through such discontinuous portions. Therefore, to suppress leakage and entry of electromagnetic waves, electrically conductive gaskets are generally inserted into the discontinuous portions of shield boxes.
To enhance the shielding property of a shield box, the shield box needs to have a structure in which a desired electric current can be passed through the entire shield box. However, such portions where metal members and gaskets are in contact with each other generally have a low contact pressure and thus have poor electrical continuity (hereinafter, simply referred to as “continuity”) between the metal members and gaskets. Thus, the amount of current passing through the contact portions tends to become small. Accordingly, to further enhance the performance of a shield box, it is important to ensure the electrical conductivity of metal plates constituting the shield box and to ensure the continuity between the metal plates and gaskets.
Since electric and electronic devices are used in various environments today, materials constituting shield boxes are required not to corrode in usage in severe environments, that is, to have high corrosion resistance. A chromate treatment has been known as a typical method for improving the corrosion resistance (white-rust resistance and red-rust resistance) of galvanized steel sheets. Galvanized steel sheets subjected to a chromate treatment with a treatment solution mainly containing chromic acid, dichromic acid, or a salt of the foregoing have been widely used as steel sheets for household electrical appliances, steel sheets for building materials, and steel sheets for automobiles.
As described above, metal members (steel sheets) constituting shield boxes need to have high electrical conductivity and furthermore have continuity with gaskets. Although films formed on surfaces of steel sheets by a chromate treatment have lower electrical conductivity than the base steel sheets, films formed by a chromate treatment can exhibit rust resistance even when the films have a small thickness. Therefore, in surface-treated steel sheets subjected to a chromate treatment, by decreasing the thickness of films having low electrical conductivity as small as possible, electrical conductivity equivalent to that of steel sheets (without surface treatment) is achieved. As a result, continuity between the metal members and the gaskets can be sufficiently ensured and thus both rust resistance and electromagnetic shielding property can be achieved. However, due to recent global environmental issues, there is an increasing demand for employing pollution-free surface-treated steel sheets provided without using a chromate treatment, that is, chromium-free treated steel sheets.
A large number of techniques relating to chromium-free treated steel sheets have been proposed. Examples of the techniques include techniques of using the passivation effect of molybdic acid and tungstic acid belonging to group IVA as with chromic acid; techniques of using metal salts of transition metals such as Ti, Zr, V, Mn, Ni, and Co and rare-earth elements such as La and Ce; techniques based on chelating agents such as polyhydric phenolcarboxylic acid, e.g., tannic acid and S- or N-containing compounds; techniques of forming a polysiloxane film with a silane coupling agent; and techniques in combination of the foregoing.
Specific examples are as follows:    (1) a technique of forming a film with a treatment solution containing a coating agent obtained by causing an organic resin such as a polyvinylphenol derivative, an acid component, and an epoxy compound to react with one another, a silane coupling agent, a vanadium compound, and the like (e.g., PTLs 1, 2, 3, and 4);    (2) a technique of forming a film containing an aqueous resin, a thiocarbonyl group, a vanadate compound, and phosphoric acid (e.g., PTL 5);    (3) a technique of forming a film with a treatment solution containing a compound of a metal such as Ti, an inorganic acid such as a fluoride or phosphate compound, and an organic acid (PTLs 6, 7, 8, 9, 10, 11, and 12);    (4) a technique in which a composite film containing a rare-earth element such as Ce, La, or Y and a Ti or Zr element is formed, and a layer having a high oxide content is formed in a region of the film closer to the interface and a layer having a high hydroxide content is formed in a region of the film closer to the front surface (PTL 13), and a technique of forming a composite film composed of Ce and a Si oxide (PTL 14);    (5) a technique of forming an organic composite coating constituted by a lower layer that is a phosphoric acid and/or phosphate compound film containing an oxide and an upper layer that is a resin film (e.g., PTLs 15 and 16); and    (6) a technique of forming a composite film composed of a specific inhibitor component and a silica/zirconium compound (e.g., PTL 17).
The films formed by these techniques are intended to suppress the generation of white rust of zinc through composite addition of organic components or inorganic components. For example, in the techniques (1) and (2), corrosion resistance is ensured by mainly adding an organic resin. However, in a film containing such an organic resin, the organic resin has an insulating property. Therefore, a steel sheet having such a film formed thereon does not have sufficient electrical conductivity and thus is not suitable as a material of shield boxes.
The techniques (3) and (4) provide films that are completely free from organic components and are composed of inorganic components only. However, such a composite film composed of a metal oxide or a metal hydroxide needs to have a large thickness to impart sufficient corrosion resistance to a galvanized steel sheet. In addition, a surface of a galvanized steel sheet is covered with a non-conductive film (insulating film) composed of zinc phosphate or the like. Therefore, as in the techniques (1) and (2), high electrical conductivity is less likely to be achieved and it is difficult to achieve both high corrosion resistance and electrical conductivity.
The technique (5) focuses on the fact that the electrical conductivity of a surface of a surface-treated steel sheet depends on the thickness of an insulating film formed on the surface, and high electrical conductivity is achieved by decreasing the thickness of the insulating film. However, the decrease in the thickness results in degradation of corrosion resistance of the steel sheet. Therefore, it is difficult to provide a surface-treated steel sheet that is excellent in terms of both corrosion resistance and electrical conductivity.
The technique (6) provides high corrosion resistance by using the passivation effect of a vanadate compound serving as an inhibitor component and a sparingly soluble metal salt formed with a phosphate compound serving as an inhibitor component, and by forming a composite film containing a zirconium compound, silica fine particles, and a silane coupling agent that constitute the skeleton of the film. However, to ensure electrical conductivity, the film thickness needs to be small. Thus, it is difficult to achieve both high corrosion resistance and electrical conductivity.
As described above, to make the chromium-free treated steel sheet having been developed so far have corrosion resistance equivalent to that of existing chromate films, films having a good insulating property need to have a large thickness. Accordingly, it is difficult for such chromium-free treated steel sheets to have sufficiently high electrical conductivity. Thus, these steel sheets do not sufficiently satisfy characteristics required for steel sheets constituting shield box bodies. In addition, as described above, to enhance the shielding property of a shield box, sufficiently high continuity needs to be achieved between metal members (steel sheets) and gaskets that are in contact with each other at a low contact pressure. However, such continuity is not considered at all in any of the above-described techniques.